Canine distemper virus (CDV) belongs to the family Paramyxoviridae within the genus Morbillivirus. CDV is an enveloped single-stranded RNA virus of about 100-300 nm in diameter. The CDV virion core contains a nucleoprotein (NP) peptide that closely associated with viral RNA. A second core peptide is a phenophorylase (P). The CDV envelope contains three peptides, M protein (matrix protein) and two glycoproteins. The glycoproteins are the hemagglutinin glycoproteins (H) and a fusion (F) glycoprotein. The fusion glycoprotein is degraded into smaller sub-units designated F.sub.1 and F.sub.2. The H protein is responsible for viral adsorption to target cells and the fusion glycoprotein is responsible for the cell-to-cell fusion. As far as is known, all distemper virus isolates contain these common viral polypeptides. The route of infection to the dog is by infective aerosol droplets. Transmission of the virus is facilitated by coughing and sneezing and close confinement in a warm, humid, closed environment. Studies suggest that viral infection occurs first in the respiratory epithelium of the upper oronasal tract with subsequent spread to the deep pulmonary parenchyma. Gorham "Canine Distemper", Advance Veterinary Science, Brandley and Jungher Editors, 6:288- 315 (1960).
Tissue macrophages and monocytes located in or along respiratory epithelium in tonsils appear to be the first cell type to pick up and replicate CDV. The virus then is spread by lymphatics in blood to distant lymphoreticular tissues. This is accomplished by viremia and occurs anywhere from two to four days after initial infection. Between eight and nine days after infection, the virus spreads beyond lymphoreticular tissues to involve epithelial and mesenchymal tissues. Appel, "Pathogenesis of Canine Distemper", The Am. J. Vet. Res., 30:1167-1182 (1969). This probably occurs in every animal regardless of the outcome of the disease process. It is at this stage of the virus infection that specific host immune responses to viral antigens influence the outcome of disease. The acute fatal form of the disease is characterized by unrestricted viral spread to virtually every tissue in the body. Virus is found in every excretion and secretion and, by using immunofluorescence methods or antigen tracing techniques, antigen can be demonstrated in virtually every cell type within the dog. For most of these animals, the most likely cause of death is fulminant fatal neurologic involvement.
Some CDV infected dogs exhibit clinically delayed progression of disease and modest convalescent immune responses. Clinical signs, if present, are subtle early in the disease and are a reflection of viral persistence within the central nervous system (CNS). Subsequent development over CNS disease is variable. Most CDV infected dogs exhibit essentially no overt clinical signs of disease and are recognized as convalescent, clinically normal dogs. Actively infected dogs destined to recover from CDV infection will demonstrate free circulating anti-viral antibodies on or about post infection day six or seven. Krakowka, et al., "Serologic Response to Canine Distemper Viral Antigens in Gnotobiotic Dogs Infected with R-252 Canine Distemper Virus", J. Infect. Dis., 132:384-392 (1975). Titers rise rapidly to high levels in early convalescence.
Dogs affected acutely with CDV show variable degrees of depression, anorexia, and fever. The skin may be variably dehydrated, dry-roughened, and inelastic. A proportion of these animals will show photophobia and some will have evidence of mucopurulent ocular-nasal discharge. Intermittent diarrhea is a common clinical sign. During this acute viremic phase of the disease, virus is shed in every secretion and excretion. As the disease progresses, pneumonia, frequently due to secondary bacterial invaders, may develop. Dogs in this stage of disease are moderately to severely lymphopenic. They frequently exhibit a coincidental leukocytosis and left shift, depending on the degree or amount of secondary infection.
Although acutely affected dogs can show virtually every combination of neurological signs, in its most common presentation a dog presents petit mal or grand mal siezures. These convulsive episodes occur over time and with increasing frequency.
The second neurologic form of canine distemper is that which occurs with old dog encephalitis (ODE), or occurs after sub-clinical infection and apparent recovery. The CNS signs can be extremely varied in presentation and can be mistaken for brain tumor, head trauma, bacterial menengitis, hydrocephalus, and spinal cord disk disease. A major non-neural manifestation of CDV infection in dogs is CDV-associated immunodepression. Krakowka, et al., "Canine Distemper Virus: A Review of Structural and Functional Modulations in Lymphoid Tissues", Am. J. Vet. Res., 41:284-292 (1980). Many of the signs of canine distemper virus infection are attributable to coincidental secondary infectious processes occurring in this debilitated animal.
The chief and most important bacterial pathogens associated with disease in dogs are the pneumonic bacterial species including: Bordetella bronchiseptica, Pasteurella spp., and of course, Staphylococcus and Streptococcus spp. These agents are responsible for the purulent conjuctivitus, rhinitis, and bronchopneumonia noted clinically in CDV-infected dogs. Mixed viral infections, chiefly of the respiratory type, also are common. In addition to canine adenovirus II infection, reovirus, canine parainfluenza virus, and presumably other viruses such as canine herpes virus, are all involved in this dual or multiple mixed infection problem.
As a consequence of direct CDV infection either in lymphoid cells and/or macrophages, lymphocytes from CDV-infected dogs are rendered incapable of generating effective in vitro and in vivo immune responses. The early virolytic effects of CDV upon the lymphoid system and macrophages also has the effect of suppressing established immune responses and/or normal host defenses.
There exists a number of safe and efficacious, modified-live viral vaccine products. The virus, consisting chiefly of laboratory adapted and attenuated Rockborn or Lederle strains of CDV, is propagated in canine origin continuous cell lines. One innoculation into the CDV-susceptible clinically healthy, normal dog, will provide effective and probably lifelong immunity. Thus, under typical clinical circumstances, immunity can be assured by delivering the product according to manufacturer's instructions to the animal or animals in question. There are, however, a number of different exceptions to this generality that should be mentioned. For example, the use of modified-live CDV vaccine in exotic animal species should be avoided. Montali, et al., "Clinical Trials with Canine Distemper Vaccines in Exotic Carnivores", J. Am. Vet. Med. Assoc., 183:1163-1167 (1983). Some exotics such as the lesser panda and black-footed ferret are highly susceptible to CDV and have been killed with modified-live (MLV) CDV vaccine. Even species within the Canidae family may show heightened susceptibility to CDV. For example, MLV-CDV was recently shown to be dangerous for use in grey foxes but not in red foxes. Halbrooks, et al, "Response of Grey Foxes to Modified-live Virus Canine Distemper Vaccines", J. Am. Vet. Med. Assoc., 179:1170-1174 (1981).
The final precaution relates to the clinical interaction of canine parvovirus infection with canine distemper virus infection. On a clinical basis, it has been noticed that canine parvovirus infection potentiates the lethal effects of either concurrent CDV infection or CDV fatalities associated with vaccination with an ordinarily safe modified-live product. Though this is a clinical association, it is known that dogs actively infected with virulent canine parvovirus are at risk for developing fatal CDV-vaccine associated neurologic disease. Krakowka, et al, "Canine Parvovirus Infection Potentiates Canine Distemper Encephalitis due to Modified-live Virus Vaccine", J. Am. Vet. Res. Assoc., 180:137-139 (1982).
Thus, there is a current need for an efficacious non-infective CDV vaccine product that can be used in combination with the ML-CPV vaccine. It is to such a vaccine that the present invention is directed.